[84LX89GT]

The stock headers are better than the usual log type manifolds because of the direction of flow they have and the scavenging....they look like they were bent by a junior high shop student with a standard tube bender, aftermarket ones are much better, but being stock i guess it's better than what they had before.
The hatch from any '79 to '93 mustang will fit and the aftermarket ones i've heard are pretty easy to mount up.
Also the intake manifold flowed less, the throttle body was smaller as mentioned 58mm, which i believe is the diameter of the mass air flow sensors in '89 - '93 mustangs....just some useless fact
Also the pistons didn't have valve reliefs and the mustangs had the "high swirl" inefficient cylinder heads that came on most of the passanger vehicles. If you want to put in a big cam or larger valves, chances are you'll have to have reliefs machined into the pistons. Those heads basically can go in the garbage, they flow horrible and i've heard that they swirl the air/fuel so much to the point where the fuel gets slung out of suspension and alot of it ends up on the cylinder walls where it cools down and doesn't get burnt. Getting a set of E7TE castings from '87-'93 mustangs are actually a big improvement.
now the positives, it's the first year of multi-port fuel injection and as such get better gas mileage, faster throttle response and more lowend torque than previous models with carburetion and throttle body fuel injection (CFI). Those cars are still fun and in my opinion are the best looking mustangs of the early to mid 80's, with the '82 GT's coming in 2nd.
I hope this helped a little bit and i also hope that it's accurate as well.